This invention relates generally to a thermosensitive recording material having a thermosensitive coloring layer and, more specifically, to a thermosensitive recording material having a thermosensitive coloring layer containing a leuco dye and a developer and a protective layer provided on the thermosensitive coloring layer. The present invention is also directed to a coating composition for forming the above protective layer, and to a method of production of the protective layer using the coating composition.
With diversification of information and expansion of needs therefor in recent years, various types of recording materials have been developed and put into practice in the field of information recording. Especially, thermosensitive recording materials are widely used in various fields such as information processing (output of a desk-top calculator, computer or the like), recorder for a medical measurement device, low- or high-speed facsimile, automatic ticket machine (railway ticket, admission ticket or the like), thermal copying machine, label of a POS system because of the following advantages:    (1) an image can be recorded only by applying heat;    (2) recording can be conducted with a simple mechanism, so that it is possible to downsize the device, and a recording material is easy to handle and inexpensive; and    (3) a recording material consists of one component of a thermosensitive paper.
Under such circumstances, a recording material which develops a color quickly at a high density, which is capable of producing an image with high durability and which has a recording surface having such high stability as to have no background fogging. In recent years, thermosensitive recording materials are also used in a large quantity in fields in which recorded images with high reliability are required such as label printing. It is unavoidable for such recording materials to be contacted with a plasticizer or oils and fats contained in an organic polymeric material used in a wrapping material, so that a thermosensitive recording material having resistance to such chemicals is also demanded.
In order to meet the above requirements, a protective layer has been conventionally provided on a thermosensitive recording layer. However, when used as a label or the like, the recording material has opportunities to be exposed to water, and water adhered to the recording material causes lowering of image strength, background fogging and water-peeling.
For the purpose of overcoming the drawback, it is suggested that polyvinyl alcohol (which will be hereinafter referred to as “PVA”) or a modified PVA, or a mixture of PVA or a modified PVA and a waterproofing agent be used in a protective layer. Water resistance is generally obtained by crosslinking the PVA or the modified PVA.
As one method of obtaining water resistance, Japanese Laid-Open Patent Publication No. H08-151412 suggests using a PVA having a diacetone group together with a hydrazine compound. However, when the PVA composition is used in a protective layer coating liquid, a “waterproofing reaction” (crosslinking reaction) occurs in the coating liquid and the viscosity of the coating liquid increases with a lapse of time.
For the purpose of overcoming this problem, Japanese Laid-Open Patent Publication No. H11-314457 proposes a thermosensitive recording body having a protective layer containing a diacetone-modified PVA and a thermosensitive coloring layer containing a hydrazine compound. However, such a thermosensitive recording material still has problems of insufficient water resistance of the protective layer, an increase in viscosity of a coating liquid for the thermosensitive coloring layer, hindrance of color developability of the thermosensitive coloring layer by the hydrazine compound and color development of the thermosensitive coloring layer coating liquid, namely, background fogging.
Japanese Laid-Open Patent Publication No. H10-87936 proposes a PVA composition comprising a PVA copolymer containing diacetone acrylamide monomer units, a hydrazine compound and a water-soluble amine. However, when this PVA composition is used in a protective layer coating liquid of a thermosensitive recording material, the amine adversely affects a thermosensitive coloring layer at the time of coating and during storage and causes background fogging. Also, it is difficult to control the pH of the protective layer coating liquid with the amine, and the amine may increase the viscosity of the protective layer coating liquid, depending upon the amount added.